Tiltrotor aircraft typically include multiple propulsion assemblies that are positioned near outboard ends of a fixed wing. Each propulsion assembly may include an engine and transmission that provide torque and rotational energy to a drive shaft that rotates a proprotor system including a hub assembly and a plurality of proprotor blades. Typically, at least a portion of each propulsion assembly is rotatable relative to the fixed wing such that the proprotor blades have a generally horizontal plane of rotation providing vertical lift for takeoff, hovering and landing, much like a conventional helicopter, and a generally vertical plane of rotation providing forward thrust for cruising in forward flight with the fixed wing providing lift, much like a conventional propeller driven airplane. In addition, tiltrotor aircraft can be operated in configurations between the helicopter flight mode and the airplane flight mode, which may be referred to as conversion flight mode.
Physical structures have natural frequencies of vibration that can be excited by forces applied thereto as a result of operating parameters and/or environmental conditions. These frequencies are determined, at least in part, by the materials and geometrical dimensions of the structures. In the case of tiltrotor aircraft, certain structures having critical natural frequencies include the fuselage, the fixed wing and various elements of the propulsion assemblies. One important operating parameter of a tiltrotor aircraft is the angular velocity or revolutions per minute (RPM) of the proprotor blades, which may generate excitation frequencies corresponding to 1/rev (1 per revolution), 2/rev, 3/rev, etc. In general, proprotor systems for tiltrotor aircraft should be designed to achieve blade flap or out-of-plane frequencies and lead-lag or in-plane frequencies that are sufficiently distant from these excitation frequencies. For example, certain tiltrotor aircraft have stiff-in-plane proprotor systems with the lead-lag frequency above 1.0/rev, such as between 1.4/rev and 1.6/rev. For each proprotor blade, such stiff-in-plane proprotor systems have utilized three independent shear bearings in series and a centrifugal force bearing positioned outboard of the yoke and within the proprotor blade. It has been found, however, that this design prevents heat dissipation from the centrifugal force bearings during operations. In addition, this design precludes visual inspection of the centrifugal force bearings without blade removal. Further, this design obstructs compact blade fold options that can reduce the overall tiltrotor aircraft footprint during storage.